A conventional common proximity sensor is composed of an oscillating means, a resonance means resonating with higher harmonic waves of an oscillating frequency of the said oscillating means, a sensing electrode connected to the said resonance means and a detecting means for detecting a signal change based on an electrostatic capacity change between the said sensing electrode and a detected object, as described in a first patent publication. The said oscillating means used herein is made into a circuit oscillating at a predetermined frequency that is determined in advance. Moreover, the resonance means composed of an LC serial resonance circuit has a circuit structure that does not resonate with the oscillating frequency but resonates with the higher harmonic waves.
Consequently, if an object comes near the sensing electrode, an electrostatic capacity changes between a surface of the object and the sensing electrode, so that a detection signal changes accordingly. Therefore, it is possible to sense an access of the object by monitoring a signal change of the detection signal.
Such type of proximity sensors improve properties against a temperature change or time degradation by setting an initial value of the electrostatic capacity so as to become a value increased by a fixed amount from a value at the time of harmonizing with the resonant frequency.
The conventional proximity sensor described in the first patent publication detects the electrostatic capacity between the object and the sensing electrode. As a result, a detection distance differs depending on a size of a human or an object. Thus, it is possible that detection accuracy becomes bad with a resultant malfunction. Moreover, there is another problem that there are many malfunctions due to a change of humidity or rain around the detection object. In addition, if the aforesaid proximity sensor is disposed on an electrically conductive body of a vehicle, it detects the electrostatic capacity between the sensing electrode and a metal body earth of the vehicle. Accordingly, it is difficult for the proximity sensor to distinguish an opening/closing operation of such door from a case in which something is caught or hammed in the door or the like. Consequently, there is a problem that it is difficult to mount the sensing electrode on a metallic body of the vehicle from a practical standpoint.
On the other hand, a Theremin circuit is well known as a circuit using an oscillating frequency of about 200 KHz to 1 MHz output from an antenna. The Theremin circuit transforms a change of the oscillating frequency into a change of a sound when a hand of a human comes near an oscillator and the antenna as a part of the oscillator, thereby being using them as an electronic musical instrument. The Theremin circuit is able to detect a change of a distance as a change of a frequency if it is used as a proximity sensor that performs detection using electromagnetic waves.
However, in common Theremin circuits, if there is a human body standing or a human body existing near it, a changed portion of a condenser capacity of a human becomes a variation of the frequency in part of the oscillator. Thus, an influence of a size of the human becomes larger. Consequently, even if a material of a physical body is changed or if a size of the physical body becomes large, it is impossible to improve accuracy or precision as the proximity sensor. Moreover, it becomes impossible to detect the human body or the like at short range, since there is a large influence of the electrostatic capacity between the antenna and the human body.
In view of the above, a technique of a second patent publication radiates microwaves that underwent a wideband frequency modulation. Then, it performs mixing of a reflected received signal of such microwaves with a transmitting signal. Thereafter, it detects reflected waves of the microwaves radiated from the antenna that have a frequency and a phase corresponding to a propagated distance of the microwaves. Then, it makes a mixer output a beat signal for computing a distance to a physical body or object on the basis of a time from radiation to receiving of the reflected waves. As such signal processing, it does multiplication and summation of two orthogonal reference signals having a frequency according to the predetermined distance and the beat signal. Then, it obtains a proportion between two summations and obtains a signal corresponding to a phase of the beat signal from the proportion. Thereafter, it converts a temporal change or spatial change of the signal corresponding to the said phase into a temporal change amount or a spatial change amount of the distance to the object.
Thereby, in case of measuring a distance by used of electric waves or sound waves such as the microwaves, its beam width becomes large to such a degree that a signal representing an average distance can be stably obtained without being affected by distribution of holes or micro irregularity of a reflection body. Therefore, there is no need to carry out an averaging processing separately. Moreover, it has a characteristic that it can speed up a detection rate or speed since the processing of the received signals can be performed at high speed.    First Patent Publication: Japanese Laid Open Patent Publication No. 2001-55852    Second Patent Publication: Japanese Laid Open Patent Publication No. 2001-4741